FIGS. 1A-1D depict the frame, slot, and slot synchronization (SYNC) word sequences and SYNC word usages, respectively, for an exemplary prior art digital Time Division Multiple Access (TDMA) cellular air interface known in the art as IS-136 (see, for example, IS-136.1, Rev. A, Mar. 21, 1996 and IS-136.2, Rev. A, Feb. 12, 1996).
FIG. 1A shows that a 40 millisecond frame consists of six time slots. Slots 1-3 and 4-6 each comprise one TDMA Block. In the forward direction from a base station to a mobile station, which is a case of most interest to this invention, the frames are continuously transmitted. A given mobile station is assigned to receive in one slot per frame for a half data rate case, and is assigned to receive in two time slots for a full data rate case. FIG. 1B illustrates the format of one slot in the direction from the base station to the mobile station. The base station forms a part of a Base Station/Mobile Switching Center/Interworking function (BMI).
In particular, FIG. 1B illustrates one time slot of a Digital Control Channel (DCCH) that is transmitted by the BMI on a forward channel. The 28 bit (14 symbol) SYNC word/time slot identifier field occupies the first 28 bits of the frame, and is followed by various other fields, including a first Shared Channel Feedback (SCF) field, a first part of a DATA field, a Coded Superframe Phase (CSFP) field, a second part of the DATA field, a second SCF field, and a Reserved (RSVD) field of two bits. Of most interest to this invention is the SYNC word field.
The SYNC word/time slot identifier field is used for slot synchronization, equalizer training, and time slot identification. The six unique synchronization sequences shown in FIG. 1C are defined so as to have good autocorrelation properties to facilitate synchronization and training. Six time slot identifiers are defined, which have good cross correlation properties. Line 8 of FIG. 1D identifies the SYNC words for a channel fully assigned to full-rate users (i.e., three full-rate users per frame). Line 1 of FIG. 1D identifies the SYNC words for a channel fully assigned to half-rate users (i.e., six half-rate users per frame). Lines 2 through 7 identify the SYNC word order for a mixture of full-rate and half-rate users, such that only one SYNC word is assigned per user. Unassigned slots are indicated by the base station as half-rate user slots in the time slot identifier field. The mobile station uses its assigned SYNC word when transmitting on a Reverse Digital Traffic Channel (RDTC).
When a mobile station first comes to a digital channel it locates and then synchronizes to a DCCH. From the DCCH the mobile station is able to determine various access and other network parameters. However, when first attempting to synchronize to the DCCH the mobile station has no information as to whether the particular channel is a half-rate channel or a full-rate channel. That is, although the mobile station may make an automatic slot assignment of the DCCH (as in, by example, reselection), or it can be ordered to a slot by the base station, the slot rate (half or full) is not known. This presents a problem in selecting the correct synchronization sequences in the mobile station for correlation purposes, since in the fully assigned full-rate case only the first three synchronization sequences of FIG. 1C are used, while for the fully assigned half-rate case all six synchronization sequences are used. This problem can result in significant additional time being required to synchronize to the DCCH, which may be objectionable to some users.